Single and hooking up
With your work-all-day/party-all-night life, you can barely keep track of your cell phone, much less the name of that shag last week. You'd rather die than be tied down—by a guy or a baby—hell, you don't have health insurance.
You need: Cheap, reliable birth control and protection from sexually transmitted diseases (STDs). Latex condoms are a must. Consider backing up with the Pill, the patch, or the implant.

Between partners
You're in a dry spell, but who cares? You've got that whole group you bike with, brunch with, blog about. You're cautious about things (you don't even send personal e-mails from work). You want to be ready, but a daily pill feels like overkill—though a baby would be worse.
You need: Condoms for STD protection when the time comes. Also consider the ring or a diaphragm/cervical cap as backup.

Happy (and horny) together
You boinked on the kitchen table, on the fire escape, in the hallway—and that was just last night. Celebs stay off coke better than you stay off each other—babies would totally crash the party.
You need: Hormonal methods, like the Pill, the ring, the Mirena IUD, the implant, the patch, or injections, which allow for plenty of whenever-wherever sex.

Anxious about hormones
You're on fire: The CEO is calling you for advice and your guy is hot, horny, and mostly available. Last thing you want is 1) a kid, and 2) bloat, moodiness, or concerns about whether there's some weird, long-term hormone issue that hasn't been discovered yet.
You need: The hormone-free copper IUD (ParaGard). If you're willing to deal with a little pre-sex prep, also consider: natural family planning, a diaphragm/cervical cap, and/or condoms.

Thinking "maybe baby"
Kids are definitely on the menu, but you're not ready to order yet. If it happened by accident, no big: You and your guy have already talked about where you'd put the crib.
You need: A method that lets you potentially start baby-making tomorrow—like condoms or a diaphragm/cervical cap. Since you're not worried about failure rates, also consider the sponge or natural family planning.

Post-kids
and want to stay that way Babies? Been there, done that, and while you love the ones you have, another would be a disaster.
You need: Reliability. And if you're over 35, you probably want something with-out hormones. Sterilization is your most reliable option, but if permanence scares you, consider the ParaGard IUD.